Muscarinic acetylcholine receptors (mAChRs), which belong to the class A family of G protein-coupled receptors (GPCRs). They are widely expressed throughout the body. Five subtypes termed M1 through M5 that respond to the endogenous neurotransmitter acetylcholine (ACh) have been identified till date. They play key role in regulating the activity of many important functions of the central and peripheral nervous system including cognitive function. M1, M3 and M5 couple to Gq, whereas M2 and M4 couple via Gi/o to downstream signaling pathways and associated effector systems (Critical Reviews in Neurobiology, 1996, 10, 69-99; Pharmacology & Therapeutics, 2008, 117, 232-243). M2 and M3 are highly expressed in the periphery and are known to be involved in gastrointestinal (GI) motility and parasympathetic responses such as salivation (Life Sciences, 1993, 52, 441-448). The M1 muscarinic receptor is predominantly expressed in the brain regions such as cortex, hippocampus and amygdala which involved in cognition, and therefore selective activation of the M1 receptor would be expected to boost cognitive performance (Annals of Neurology, 2003, 54, 144-146).
Xanomeline, a muscarinic acetylcholine receptor agonist with reasonable selectivity for the M1 and M4 subtypes, produced significant effects on cognition in a clinical Alzheimer's disease (AD) trial (Alzheimer Disease and Associated Disorders, 1998, 12(4), 304-12) although gastrointestinal side effects led to a high dropout rate in clinical trials. There is a high degree of conservation between muscarinic receptor subtypes at their orthosteric acetylcholine ligand binding sites which makes it difficult to identify a M1 selective agonist.
To circumvent this issue of selectivity and safety, an alternative approach consists of developing M1 PAMs that act at the less conserved allosteric binding site. Merck reported the development of M1 positive allosteric modulator, PQCA (1-{[4-cyano-4-(pyridine-2-yl)piperidin-1-yl]methyl}-4-oxo-4H-quinolizine-3-carboxylic acid). This compound is highly selective for M1 over the other muscarinic receptor sub types and found to be efficacious in several preclinical models of cognition (Psychopharmacology, 2013, 225(1), 21-30) with no gastrointestinal side effects at doses equal to or less than a fivefold margin from the minimum effective dose required to improve cognition. In preclinical studies it was demonstrated that M1 activation increases neurotransmitter acetylcholine concentration in brain. Moreover, the M1 activation has potential as disease-modifying therapy for AD by both shifting the APP processing towards the non-amyloidogenic α-secretase pathway and by decreasing the tau hyper-phosphorylation. Positive allosteric modulators at M1 receptor have demonstrated to increase the generation of sAPPα in-vitro (The Journal of Neuroscience, 2009, 29, 14271-14286). Therefore, M1 PAMs provide an approach to target both symptomatic and disease-modifying treatment of cognitive deficits in AD and schizophrenia.
PCT patent application publications, WO2015049574, WO2015028483, WO2007067489 and WO2011149801 have disclosed some M1 PAM compounds. While several M1 PAMs have been disclosed in the literature till date, no drug acting as M1 PAM is launched in the market. Therefore, there is an un-met need and scope to discover and develop new M1 PAMs with novel chemical structures devoid of any side effects for the treatment of disorders, which are regulated by M1 receptors.